1. Technical Field of the Invention
The present invention relates generally to engine rotational speed determining devices, engine starting possibility predicting devices, engine friction estimating devices, and engine automatic stop control devices. More particularly, the invention relates to an engine rotational speed determining device, an engine starting possibility predicting device, an engine friction estimating device, and an engine automatic stop control device, which perform the respective functions, for an internal combustion engine that is started by an electric motor, based on a change in current supplied to the motor.
2. Description of the Related Art
Japanese Patent First Publication No. 2007-83965 discloses a device that determines, during a starting operation of an internal combustion engine by a starter, the rotational speed of the engine based on a change in the terminal voltage of a battery that powers the starter.
More specifically, in the vicinities of compression top dead centers of the engine, forces counteracting the rotation of a crankshaft of the engine with the starter are increased, thus decreasing the rotational speed of the engine; further, the discharge current of the battery is increased, thus decreasing the terminal voltage of the battery. Therefore, the rotational speed of the engine can be determined based on the fact that the cycle of change in the terminal voltage of the battery corresponds to the time required for an angular change of (720°/Nc) for the crankshaft, where Nc is the number of cylinders of the engine.
However, the internal resistance of the battery depends largely on both the State of Charge (SOC) of the battery and the deterioration degree of the battery. More specifically, the internal resistance of the battery is increased with decrease in the SOC of the battery; it is also increased with increase in the deterioration degree of the battery. Accordingly, the change in the terminal voltage of the battery during the starting operation of the engine also depends largely on both the SOC and deterioration degree of the battery.
Consequently, it may be difficult for the device to accurately detect the rotational speed of the engine based on the change in the terminal voltage of the battery during the starting operation of the engine.